The field of the invention is patient monitoring systems. More particularly, the invention relates to a blood pressure monitoring method and system for determining pulse rate and blood pressure of a patient.
The heart muscles of humans periodically contract to force blood through the arteries. As a result of this pumping action, pressure pulses exist in these arteries and cause them to cyclically change volume. The baseline pressure for these pulses is known as the diastolic pressure and the peak pressure for these pulses is known as the systolic pressure. A further pressure value, known as the “mean arterial pressure” (MAP), represents a time-weighted average of the blood pressure. The systolic, MAP and diastolic values for a patient are useful in monitoring the cardiovascular state of the patient, in diagnosis of a wide variety of pathological conditions, and in treating disease. Therefore, it is a great advantage to a clinician to have an automatic device which can accurately, quickly, and non-invasively estimate these blood pressure values.
There are different techniques and devices for measuring one or more of these blood pressure values. One method in particular involves applying an inflatable pressure cuff about the upper arm of a human and inflating it above systolic pressure so as to restrict the flow of blood in the brachial artery. The pressure is then slowly relieved while a stethoscope is used on the distal portion of the artery to listen for pulsating sounds, known as Korotkoff sounds, which accompany the reestablishment of blood flow in the artery. As the pressure in the cuff is reduced further, the Korotkoff sounds change and eventually disappear. The cuff pressure at which the Korotkoff sounds first appear during deflation of the cuff is an indirect measure of the systolic pressure and the pressure at which these sounds disappear is an indirect measure of the diastolic pressure. This method of blood pressure detection is generally known as the auscultatory method.
Another method of measuring blood pressure is referred to as the oscillometric technique. This method of measuring blood pressure involves applying an inflatable cuff around an extremity of a patient's body, such as the patient's upper arm. The cuff is then inflated to a pressure above the patient's systolic pressure and then reduced over time while a pressure sensor measures the cuff pressure. The sensitivity of the sensor is such that pressure fluctuations within the cuff resulting from the beats of the patient's heart may be detected. With each beat there is a resulting small change in the artery volume, which is transferred to the inflated cuff causing slight pressure variations within the cuff that are detected by the pressure sensor. The pressure sensor produces an electrical signal showing the cuff pressure and a series of small periodic variations associated with the beats of a patient's heart. It has been found that these variations, called “complexes” or “oscillations,” have a peak-to-peak amplitude which is minimal for applied cuff pressures above the systolic pressure and below the diastolic pressure. As the cuff pressure is decreased from a level above the systolic pressure the oscillation size begins to monotonically grow and eventually reaches a maximum amplitude. As the cuff pressure continues to decrease past the oscillation maximum the oscillation size decreases monotonically. Physiologically, the cuff pressure at the maximum value approximates the MAP. In addition, the complex amplitudes of cuff pressures equivalent to the systolic and diastolic pressures have a relationship to this maximum value that is dependent on arterial compliance. In the majority of the population, this relationship can be approximated by a fixed ratio. Thus, the oscillometric method is based on measurements of detected complex amplitudes at various cuff pressures.
Blood pressure measuring devices operating according to the oscillometric method detect the peak-to-peak amplitude of the pressure complexes at various applied cuff pressure levels. The amplitudes of these complexes, as well as the applied cuff pressure, are stored together as the device automatically changes the cuff pressures over a range of interest. These peak-to-peak complex amplitudes define an oscillometric “envelope” and are evaluated to find the maximum value and its related cuff pressure, which is approximately equal to MAP. A cuff pressure below the MAP value that produces a peak-to-peak complex amplitude having a certain fixed relationship to the maximum value, is designated as the diastolic pressure. Likewise, a cuff pressure above the MAP value that results in complexes having an amplitude with a certain fixed relationship to that maximum value, is designated as the systolic pressure. The ratios of oscillation amplitude at the systolic and diastolic pressures to the maximum value at MAP, are empirically derived and assume varying levels depending on the preferences of those of ordinary skill in the art. Generally, these ratios are in the range of 40% to 80%.
One way to determine estimates of blood pressure is to computationally fit a curve to the oscillometric envelope defined by the complex amplitude versus cuff pressure data points which are measured by a blood pressure monitor during a determination. The fitted curve may then be used to compute an estimate of the MAP value, which is approximately at the maximum value of the fitted curve and is therefore easily determined by finding the point on the fitted curve for which the first derivative equals zero. From this maximum value data point, the systolic and diastolic pressures may be computed by finding fixed percentages of the maximum complex amplitude on the curve and using the associated cuff pressure levels as the systolic and diastolic estimates. In this manner, indirect estimates of the systolic, MAP, and diastolic arterial pressures may be found and ultimately output by an oscillometric device. The curve fitting technique has the value of smoothing the envelope information so that artifact variations are minimized and no single point dominates in the calculation of blood pressure. This results in more accurate estimates. The curve fit may also be stored for future use in estimating complex size at a given pressure level.
However, the reliability and repeatability of these computations hinges more significantly on the ability to accurately determine the magnitudes of the oscillation complexes. There are several barriers to accurate and reliable oscillation magnitude determination. First, artifacts caused by patient motion and other effects are often present. These artifacts are superimposed upon the desired oscillometric signal, causing it to be distorted. Second, the typical oscillometric non-invasive blood pressure monitor will use a band-pass filtered channel to detect and measure pulses. While this band-pass filter has the good effect of removing significant amounts of noise, it can distort the needed and true physiological components of the oscillometric signal. For example, the cut-off frequency of the high-pass portion of the band-pass filter must be set to help remove low frequency artifact, yet this same filter will also remove signal frequencies which resulted from the heart beat. This distorts the signal causing errors in measurement. Therefore, there exists the need for a system and method of effectively discriminating between true and erroneous pulse data using pulse quality values and dual channel signal processing.